Every type of cell maintains a specific size to best perform its physiological functions. Red blood cells must be small enough to squeeze through capillaries, whereas macrophages must be large enough to engulf pathogens. Although many key regulatory proteins affecting cell size are known, and cell size control is both important and easily observable in many species, the molecular mechanisms underlying size control remain poorly understood. Controlling cell size during proliferation requires tying division to growth. We will investigate the question of how growth can trigger proliferation in budding yeast, which are an ideal model because they proliferate rapidly, allow live-cell size measurements, and lack the complications of multi-cellularity. We also aim to apply our framework for studying size control to mouse and human cells. In budding yeast, cell size control occurs in G1 phase of the cell cycle. The sensitivity of cell size to the dosage of CLN3, a G1 cyclin that acts in complex with the cyclin-dependent kinase Cdk1, indicates that Cln3 is a central component of the cell size control mechanism. However, Cln3 concentration remains nearly constant in G1, so the mechanism through which it would trigger division has been unclear. Rather than cell size increasing Cln3 activity, we show that cell growth dilutes Whi5, a rate-limiting inhibitor of progression through G and the primary target of Cln3-Cdk1 complexes. Importantly, while the synthesis of Cln3 scales with cell size so that its concentration remains constant, the synthesis of Whi5 does not scale with cell size so that all cells produce approximately the same amount of Whi5. This results in a higher Cln3-Whi5 ratio in larger cells that drives proliferation. Here, we aim to perform a comprehensive series of experiments using quantitative live cell microscopy, image analysis and signal processing, in vitro and in vivo biochemistry, yeast genetics, CRISPR/Cas9 genome editing, and genomic technologies to determine the molecular mechanism of size control. Specifically, we aim to 1. Determine the molecular origin of size-dependent protein synthesis; 2. Determine the molecular mechanism Cln3 uses to inhibit Whi5 and set the size threshold; and 3. Extend our size control methods and analytical framework to mammalian cells to determine if cell cycle inhibitor dilution is a mechanism tying growth to proliferation in human cells. Relevance to human health: Our investigation of cell size control may provide important insight into human G1/S regulation, which is necessarily misregulated in cancer.